Japanese Unexamined Patent Application Publication (“Kokai”) No. H11 (1999)-353587 discloses a navigation apparatus comprising: a storage device for storing information pertaining to a radio station that broadcasts traffic information, such as the location of the radio station, coverage, and reception frequency; a receiver for receiving the traffic information; a control section for operating the receiver provided that both the receiver and the location of the vehicle, detected by a position detecting device, are located within the coverage of the radio station; and an information conversion device for analyzing the traffic information received by the receiver as audio data and converting the traffic information to textual information; wherein the textual information from the information conversion device is displayed on a monitor screen 2. In the disclosed navigation apparatus, road information received by a tuner section of the receiver is output from a speaker as audio guidance while the road information is analyzed by an audio recognition section of the information conversion device and displayed on the monitor screen after being converted to textual information.
In operation of the navigation apparatus disclosed in Japanese Kokai H11-353587, even if there is no congested area on a travel route while the vehicle is located within the coverage of the radio station for which data is stored in the storage device, traffic information for the travel route is automatically announced with audio guidance. Therefore, when the driver of the vehicle is listening to music, traffic information unnecessary for the driver interrupts the music. What is worse is that unnecessary audio information might decrease the driver's alertness during his/her driving, so that the driver might feel stressed. Further, the receiver in the navigation apparatus cannot receive traffic information such as traffic control information, road construction information, and/or traffic accident information after the vehicle has moved out of the area of coverage of the radio station for which data is stored. Therefore, the driver cannot obtain traffic information regarding a congested area on the travel route ahead of the vehicle.